There are some controls for IPv6 in the 102 firmware in the rt-n66u, though. I don't know what was there before that. I also don't know what the complete set of controls that needs to be implemented would consist of.
On the other hand, there are no references to IPv6 in the rt-n56u firmware. There is an open issue in "Padavan's firmware" for adding IPv6 (issue #69), but no comments on the issue have been added for a couple of weeks.
The custom firmware for the rt-n56u pointed to above does not yet support IPv6. It may at some point, but it doesn't yet. There is an outstanding issue to add this support (issue #69), but it isn't there now.
I can't recommend this bug-fixed firmware for the rt-n56u enough, though, despite the lack of IPv6 support.